DESCRIPTION: This new proposal is concerned with the role of the cdk inhibitor p27Kip1 (p27) in regulating entry into and exit from quiescence in the murine fibroblast cell cycle. Data presented document that platelet-derived growth factor (PDGF), a growth factor that causes quiescent cells to become competent for cell cycle traverse, is able to repress p27 synthesis in murine fibroblasts. The research proposed in this application will test the hypotheses that (1) PDFG regulates the production of p27 by a translational control mechanism and (2) in concert with plasma factors, PDGF also regulates the degradation of p27. This ability of PDGF to regulate the total amount of p27 modulates cyclin/cdk activity and subsequently controls cell cycle exit and entry. In order to test these hypotheses and determine the growth factor regulation of p27 activity, four specific aims are proposed. The first specific aim is designed to define and characterize the cis acting translational control element(s) in the p27 mRNA, whereas the second aim will isolate and characterize the trans activity responsible for PDGF mediated inhibition of p27 synthesis. The third specific aim will investigate the biological function and cell cycle mediated control of this transacting factor. The final aim will examine the mechanism through which multiple growth factor signals collaborate to bring about p27 degradation in mid to late G1 and identify the elements that target p27 to this degradative pathway.